"The majority of people underestimate by about 10 years the reproductive lifespan of men and women," says researcher, Dr Karin Hammarberg, of the Jean Hailes Research Unit at Monash University.

Her study also found that many people do not know when in a woman's menstrual cycle she is most likely to get pregnant.

It has been known for a long time that couples are delaying childbearing, that obesity is increasing and about 1 in 5 people of reproductive age still smoke.

Today about 9 per cent of couples have difficulty conceiving and about 35,000 women per year in Australia and New Zealand go through IVF treatment.

"These are lifestyle factors that would influence the chance of having a baby - the chance of conceiving in the first place - but also the chance of the pregnancy being complication-free and the baby being born healthy," says Hammarberg.

The new survey was carried out with support from the Commonwealth-funded Fertility Coalition, which is interested in people making informed choices about fertility, to find out whether people know these facts about their fertility.

"It's the first time anyone has surveyed what people in general know about these things," says Hammarberg.

She and colleagues conducted 462 telephone interviews of Australians aged 18 to 45 who wished to have a child now or in the future.

Only 1 in 4 correctly identified that female fertility starts to decline before age 35, while only 1 in 3 correctly identified that male fertility starts to decline before age 45.

"The majority were about 10 years out," says Hammarberg.

More than a quarter of respondents thought age did not influence male fertility or didn't know whether male fertility was affected by age.

"People were very optimistic about fertility being constant in a man," says Hammarberg.

Lifestyle factors

Most survey respondents were aware that female obesity and smoking affected fertility, but fewer recognised these factors also affected male fertility (30 per cent for obesity and 36 per cent for smoking).

Hammarberg says people need to realise both men and women need to lose weight and give up smoking to maximise the chance of conception.

"Fertility has always been cast as a woman's issue but we really want to draw the men into this conversation and make sure it's perceived as a couple issue," she says.

"It has to be a joint venture where both optimise their chances."

Knowledge about menstrual cycle

Perhaps the most surprising results of the survey were that almost 40 per cent of respondents were ill informed about when in the menstrual cycle a woman is most likely to conceive.

"It's a bit staggering," says Hammarberg.

She says 32 per cent correctly identified the middle week between periods was the time when a woman was most likely to conceive.

29 per cent said the week just after her period - which could correspond to the fertile window, says Hammarberg.

However, she says, 18 per cent incorrectly said a woman was most likely to conceive in the week just before the next period, 7 per cent said it could happen any day except during the woman's period, and 2 per cent said a woman was most likely to conceive during her period. 12 per cent said they didn't know.

These findings support those from previous studies, says Hammarberg.

"The understanding of the actual physiology and the biology of the menstrual cycle is quite poor - even among women who try to get pregnant," she says.